Each handle on a rebar is defined by a plane, and can be constrained along the direction perpendicular to the plane. Rebar constraints work by locking the handle planes to planar references, or 'targets.' These targets can be:

1. planar surfaces of host elements, or
2. the handle planes of stirrup bars (only applies to standard style bars).

For planar host element surfaces, a rebar handle can either be locked at a constant distance, or, if the host surface has a specified cover, then the handle can be joined directly to the cover of the surface.

Standard style bars can be locked to the handle planes of stirrup style rebar. In the special case of a straight, standard style bar, running perpendicular to the plane of the stirrup bar, the bar can constrain itself to distinct locations along bends in stirrup bars - points located at 0 degrees, 45 degrees, 90 degrees, etc. around each bend. This is done by simultaneously locking both the straight bar's edge handle and its planar position handle to one or both of the stirrup edges adjacent to the bend in the stirrup.

Usually, to form a constraint, the handle plane and the reference plane must be parallel. However, bar end handles can be constrained to planes at angles up to 60 degrees. Arc-shaped rebar is a special case, and can form constraints to concentric host surfaces.

RebarConstraints can only be constructed internally by Revit. They are available to the API by querying a rebar element's RebarConstraintsManager.